Images outputted from electrophotographic copying machines or printers have far poorer quality than lithographic print or silver salt system photograph. Thus, various improvements have been made both in image forming apparatus and powder toner used therefor.
For the part of toner, it has recently been necessary more and more to reduce the particle diameter of particulate toner in order to improve image quality such as resolution. Various technical developments have been made.
However, most of powder toners for development of electrostatic image commercially available at present have a volume-average particle diameter of from about 8 to 13 .mu.m. Powder toners having the smallest particle diameter have a volume-average particle diameter of about 7 .mu.m (as measured by means of Coulter Multisizer). Thus, the smallest allowable particle diameter of particulate toners extremely useful for the enhancement of image resolution is about 7 .mu.m at present. No particulate toners having smaller particle diameters are commercially produced. Little or no developing machines using such a small particle size toner have been developed.
It has thus been keenly desired to provide a toner having an even smaller particle diameter and an excellent triboelectricity and develop a development process using such a toner.
A powder toner is prepared by a dry process such as pulverization method or a wet process such as polymerization process and so-called phase inversion emulsification method as described in JP-A-5-66600 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). It is said that the smallest allowable particle diameter of toners produced by a pulverization method using the present crushing machine on an industrial basis is about 7 .mu.m. Of course, small particle diameter toners having a particle diameter of about 5 .mu.m can be produced. However, these toners cannot hardly be considered practical because they add to cost and exhibit deteriorated triboelectricity or powder fluidity caused by the reduction of the particle diameter thereof.
The wet process such as polymerization process and emulsification process is said to be essentially free from difficulty caused by the reduction of the toner particle diameter. However, the prior art wet process toner is mainly intended in the stage of development or production to replace the foregoing pulverization process toner having an ordinary volume-average particle diameter range (about 7 to 13 .mu.m). Small particle diameter toners having a volume-average particle diameter of not more than about 6 .mu.m have been so far known only in fragments. No practical formulations have been known.
On the other hand, for the part of image forming apparatus, studies have been made to cope with the reduction of the particle diameter of toners for the purpose of improving image quality. As mentioned above, however, toners having a sufficiently small particle diameter cannot be produced invariably. Therefore, for the part of image forming apparatus, a complete image forming method corresponding to such a small particle diameter toner cannot be developed. Accordingly, an image forming method which allows the formation of high resolution image and is adapted for a small particle diameter toner having a volume-average particle diameter of not more than about 6 .mu.m has never been sufficiently established.